Does light travel through a hole in the middle? If the fiber is solid glass, how does the light “squeeze” through it?
Optical fiber is not hollow; it is a solid filament of high-purity glass (silica). The mechanism that allows light to travel through this solid medium without “squeezing” or being blocked is a fundamental principle of optics called Total Internal Reflection (TIR).
1. The Physical Structure
An optical fiber consists of three main solid layers:
- Core: The innermost part made of extremely pure glass with a slightly higher refractive index.
- Cladding: A layer of glass surrounding the core with a slightly lower refractive index.
- Coating: A protective polymer layer (like polyacrylate or polyimide) to provide mechanical strength.
For example, a standard fiber like the OFSCN® G.652D Optical Fiber has a 9µm core and a 125µm cladding, both of which are solid glass.
2. How Light Travels (The Physics)
Light does not need a “hole” to travel. In fact, light travels through many solid or liquid transparent materials (like water or glass).
In an optical fiber, when light enters the core at a specific angle, it hits the boundary between the Core and the Cladding. Because the cladding has a lower refractive index, the light is reflected back into the core rather than passing through the cladding. This process repeats thousands of times per meter, guiding the light down the fiber even if the fiber is bent.
3. Application in Sensing
Because the fiber is solid, we can “write” microscopic structures directly into the glass core using lasers. These are called Fiber Bragg Gratings (FBG). Since the glass is solid, these gratings stay fixed in place and change their reflected wavelength based on physical changes like temperature or stretching.
You can see examples of these solid-state sensors here:
- OFSCN® Standard Femtosecond Fiber Bragg Gratings / Fiber Bragg Grating Strings (Bare)
- OFSCN® 300℃ Small diameter optical fiber
Visual Reference
Below is an image of a standard solid optical fiber used in these technologies:
